Answer to "What GPU's are compatible with iMac 27" 2011?"Hi all, i have an 2010 21,5” iMac with a faulty GPU. I found a cheap GTX 260 from an Alienware Notebook. I have put this GPU into my iMac, i hear the Startup-Sound, but i got only the first two LED’s light up. Is this normal, if the GPU doen’t have Apple Firmware on it ? No screen anyway. I have tried High Sierra so far, but no luck. I will try El Capitan/Sierra.